The present invention relates generally to displays, and more particularly to liquid crystal displays.
Visual display devices may utilize liquid crystals. The property of liquid crystals that enables them to be used in visual displays is the ability of liquid crystals to transmit light in a strictly aligned or field-on state, and to scatter light and/or to absorb it especially when combined with an appropriate dye, in a relatively free or field-off state. An electric field may be selectively applied across the liquid crystals to switch between field-off and field-on states.
It is desirable that liquid crystal visual displays have excellent contrast between the characters displayed and the background and high brightness in all ambient light conditions. It is also desirable that the display be free of front surface glare.
There are three categories of liquid crystal materials, namely, cholesteric, nematic and smectic. The present invention relates in a preferred embodiment described hereinafter to the use of a liquid crystal that is nematic or operationally nematic. By "operationally nematic" is meant that, in the absence of external fields, structural distortion of the liquid crystal is dominated by the orientation of the liquid crystal at its boundaries rather than by bulk effects, such as very strong twists (as in cholesteric material) or layering (as in smectic material). Thus, for example, a liquid crystal material including chiral ingredients which induce a tendency to twist but which cannot overcome the effects of the boundary alignment of the liquid crystal material would be considered to be operationally nematic. A more detailed explanation of operationally nematic liquid crystal material is provided in co-pending U.S. patent application Ser. No. 477,242, filed Mar. 21, 1983, now U.S. Pat. No. 4,616,903, in the name of Fergason, entitled ENCAPSULATED LIQUID CRYSTAL AND METHOD, assigned to Manchester R&D Partnership, the disclosure of which is hereby incorporated by reference. Reference may also be made to U.S. Pat. No. 4,435,047, issued Mar. 6, 1984, in the name of Fergason, entitled ENCAPSULATED LIQUID CRYSTAL AND METHOD, assigned to Manchester R&D Partnership, which disclosure is also hereby incorporated by reference.
The operationally nematic liquid crystal is contained in a containment medium that forms volumes of liquid crystal material. In the field-off condition, or any other condition which results in the liquid crystal being in a distorted or randomly aligned state, the liquid crystal structure is distorted to a curved form (hence curvilinearly aligned) wherein the spatial average orientation of the liquid crystal over a capsule-like volume, for instance, is strongly curved and there is no substantial parallel directional orientation of the liquid crystal in the absence of a prescribed input.
Preferably, the operationally nematic liquid crystal has a positive dielectric anisotropy, and has an ordinary index of refraction that substantially matches that of the containment medium. Such material will cause scattering of light incident thereon in the field-off or relatively free state. This liquid crystal material, described in detail in the above-identified U.S. Pat. No. 4,435,047, may be designated encapsulated operationally nematic liquid crystal material or nematic curvilinearly aligned phases ("NCAP") liquid crystal material.
The present invention, however, is not necessarily limited to NCAP configured liquid crystal. Certain embodiments of the invention may be employed with any of the various types of liquid crystal materials or configurations thereof that selectively scatter and/or absorb or transmit light in response to a prescribed input.
Usually liquid crystal is anisotropic both optically and, for example in the case of nematic liquid crystal, electrically. The optical anisotropy is manifest by the scattering of light when the liquid crystal is in random alignment, and the transmission of light through the liquid crystal when it is in ordered alignment.
A pleochroic dye may be present with the liquid crystal material to provide substantial attenuation by absorption in the field-of state but to be substantially transparent in the field-on state.
Any reference to the ability of liquid crystal to scatter and/or absorb light in accordance with the present invention should not be limited to the scattering and minimal absorption properties of liquid crystal but should include the additional properties pleochroic dyes may impose on the optical properties of the liquid crystal.
The present invention relates to improvements in displays as well as to the utilization of the light scattering and absorption characteristics of liquid crystal materials in displays. The invention also relates to the use of such materials and characteristics, together with a pleochroic or diochroic dye, for example, to obtain relatively dark characters or information displayed on a relatively bright background in both small and large size displays.
An object of the present invention is to provide a display having a relatively high quality of optical brightness and contrast.
A further object of the present invention is to provide a liquid crystal display that has excellent contrast and high brightness in all ambient light conditions.
Another object of the present invention is to improve the performance of a liquid crystal display in viewing conditions where glare is present.